objtool: Make jump label hack optional

Objtool secretly does a jump label hack to overcome the limitations of
the toolchain.  Make the hack explicit (and optional for other arches)
by turning it into a cmdline option and kernel config option.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/3bdcbfdd27ecb01ddec13c04bdf756a583b13d24.1650300597.git.jpoimboe@redhat.com

arch/Kconfig

@@ -46,6 +46,7 @@ config JUMP_LABEL
 	bool "Optimize very unlikely/likely branches"
 	depends on HAVE_ARCH_JUMP_LABEL
 	depends on CC_HAS_ASM_GOTO
+	select OBJTOOL if HAVE_JUMP_LABEL_HACK
 	help
 	 This option enables a transparent branch optimization that
 	 makes certain almost-always-true or almost-always-false branch
@@ -1031,6 +1032,9 @@ config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
 config HAVE_OBJTOOL
 	bool
 
+config HAVE_JUMP_LABEL_HACK
+	bool
+
 config HAVE_STACK_VALIDATION
 	bool
 	help

arch/x86/Kconfig

@@ -212,6 +212,7 @@ config X86
 	select HAVE_IOREMAP_PROT
 	select HAVE_IRQ_EXIT_ON_IRQ_STACK	if X86_64
 	select HAVE_IRQ_TIME_ACCOUNTING
+	select HAVE_JUMP_LABEL_HACK		if HAVE_OBJTOOL
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZ4

arch/x86/include/asm/jump_label.h

@@ -20,7 +20,7 @@
 	_ASM_PTR "%c0 + %c1 - .\n\t"			\
 	".popsection \n\t"
 
-#ifdef CONFIG_OBJTOOL
+#ifdef CONFIG_HAVE_JUMP_LABEL_HACK
 
 static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
 {
@@ -34,7 +34,7 @@ l_yes:
 	return true;
 }
 
-#else /* !CONFIG_OBJTOOL */
+#else /* !CONFIG_HAVE_JUMP_LABEL_HACK */
 
 static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
 {
@@ -48,7 +48,7 @@ l_yes:
 	return true;
 }
 
-#endif /* CONFIG_OBJTOOL */
+#endif /* CONFIG_HAVE_JUMP_LABEL_HACK */
 
 static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {

scripts/Makefile.build

@@ -227,6 +227,7 @@ ifdef CONFIG_OBJTOOL
 objtool := $(objtree)/tools/objtool/objtool
 
 objtool_args =								\
+	$(if $(CONFIG_HAVE_JUMP_LABEL_HACK), --hacks=jump_label)	\
 	$(if $(CONFIG_X86_KERNEL_IBT), --lto --ibt)			\
 	$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)		\
 	$(if $(CONFIG_UNWINDER_ORC), --orc)				\

scripts/link-vmlinux.sh

@@ -117,6 +117,10 @@ objtool_link()
 		# Don't perform vmlinux validation unless explicitly requested,
 		# but run objtool on vmlinux.o now that we have an object file.
 
+		if is_enabled CONFIG_HAVE_JUMP_LABEL_HACK; then
+			objtoolopt="${objtoolopt} --hacks=jump_label"
+		fi
+
 		if is_enabled CONFIG_X86_KERNEL_IBT; then
 			objtoolopt="${objtoolopt} --ibt"
 		fi

tools/objtool/builtin-check.c

@@ -31,8 +31,28 @@ static int parse_dump(const struct option *opt, const char *str, int unset)
 	return -1;
 }
 
+static int parse_hacks(const struct option *opt, const char *str, int unset)
+{
+	bool found = false;
+
+	/*
+	 * Use strstr() as a lazy method of checking for comma-separated
+	 * options.
+	 *
+	 * No string provided == enable all options.
+	 */
+
+	if (!str || strstr(str, "jump_label")) {
+		opts.hack_jump_label = true;
+		found = true;
+	}
+
+	return found ? 0 : -1;
+}
+
 const struct option check_options[] = {
 	OPT_GROUP("Actions:"),
+	OPT_CALLBACK_OPTARG('h', "hacks", NULL, NULL, "jump_label", "patch toolchain bugs/limitations", parse_hacks),
 	OPT_BOOLEAN('i', "ibt", &opts.ibt, "validate and annotate IBT"),
 	OPT_BOOLEAN('m', "mcount", &opts.mcount, "annotate mcount/fentry calls for ftrace"),
 	OPT_BOOLEAN('n', "noinstr", &opts.noinstr, "validate noinstr rules"),
@@ -87,14 +107,15 @@ int cmd_parse_options(int argc, const char **argv, const char * const usage[])
 
 static bool opts_valid(void)
 {
-	if (opts.ibt		||
-	    opts.mcount		||
-	    opts.noinstr	||
-	    opts.orc		||
-	    opts.retpoline	||
-	    opts.sls		||
-	    opts.stackval	||
-	    opts.static_call	||
+	if (opts.hack_jump_label	||
+	    opts.ibt			||
+	    opts.mcount			||
+	    opts.noinstr		||
+	    opts.orc			||
+	    opts.retpoline		||
+	    opts.sls			||
+	    opts.stackval		||
+	    opts.static_call		||
 	    opts.uaccess) {
 		if (opts.dump_orc) {
 			fprintf(stderr, "--dump can't be combined with other options\n");

tools/objtool/check.c

@@ -1592,7 +1592,7 @@ static int handle_jump_alt(struct objtool_file *file,
 		return -1;
 	}
 
-	if (special_alt->key_addend & 2) {
+	if (opts.hack_jump_label && special_alt->key_addend & 2) {
 		struct reloc *reloc = insn_reloc(file, orig_insn);
 
 		if (reloc) {

tools/objtool/include/objtool/builtin.h

@@ -12,6 +12,7 @@ extern const struct option check_options[];
 struct opts {
 	/* actions: */
 	bool dump_orc;
+	bool hack_jump_label;
 	bool ibt;
 	bool mcount;
 	bool noinstr;